This invention relates to brushless DC fans, which is to say fans driven by brushless DC motors, and more particularly to such fans that have a circuit permitting operation of the fan over a range of voltages, allowing fan speed and air delivery to be programmed, making available temperature tracking, limiting current, and affording locked rotor protection.
Brushless DC fans are increasingly popular for such applications as cooling electronics. A fan of a given size may have more than one operating voltage. That is, a given fan size may be available in both 12 and 24 volt models. For a particular fan size of a particular operating voltage, different air flow requirements can dictate the use of different motor windings, electronics, and protection circuitry. These differing fan characteristics among fans that appear identical in size, housing layout, venturi, and impeller raise difficulties in manufacturing control and add cost as compared to the production of a single fan model with consistent windings and circuitry. Frequently, fan customers inventory several different model fans to meet the varied requirements among their varied products. Considerable simplification of manufacturing by the fan maker, and of ordering and stocking by the customer could be effected if a single standardized brushless DC fan could be provided for a particular desired fan size and input voltage and if a means of programming that fan's operating characteristics could be utilized to simply set the operational characteristics of the fan for its particular rated voltage without winding changes and without extensive circuit modifications. This also would have the benefit of enabling the manufacturer to respond quickly to customer orders either from inventories or by rapidly increasing current production without time-consuming production line changes.
In past brushless DC fans, locked rotor protection was afforded by a positive temperature coefficient (PTC) thermistor that was in series between the windings and the input connections to the fan. The thermistor was supported in close proximity to the stator on a circuit board. The circuit board, in addition, supported the commutation sensing device that controlled stator winding energization and the stator winding energization circuit that directed flow of direct current to the windings under control of the commutation sensing device. The increased temperature that arose with a locked rotor caused the thermistor resistance to increase significantly and reduce the current to the particular stator winding when, in the locked rotor condition, the impeller was immobilized.
For the purpose of limiting current to a fan during start up, or in a locked rotor condition, or for varying the voltage applied to a brushless DC fan, the inventor has, in the past, suggested an external voltage regulator. But both fan current limiting and voltage regulation by one voltage regulator were not suggested, particularly by internal fan circuitry. An integrated circuit like the Motorola LM117, LM217 or LM317 was suggested for use as a power supply for fans for cooling electronics. This circuit is a three-terminal device that has an output voltage dependent on a voltage established at an "adjust" terminal thereof. Fan customers, it was suggested, could choose a value of resistance between that terminal and ground to provide the desired air flow for cooling. The use of a thermistor as the resistance element connected to the adjust terminal of such a separate regulation circuit could be used to monitor temperature and adjust fan speed based on the monitored temperature.
In brushless DC motors it has long been recognized that the need for a commutation and energization circuit that controlled energization of the stator windings was a commercial impediment by virtue of its increasing the cost of these motors vis-a-vis AC motors or brush-type DC motors that require no such circuitry. This has been true of brushless DC fans. There has been, therefore, a continuing effort to minimize the cost of such circuitry such that fewer and less expensive circuit components have been chosen to reduce the overall cost of brushless DC fans. Moreover, competition for brushless DC fans has been primarily price competition, and this too has led away from additions to the internal fan circuitry.